The currently accepted thinking is that global warming is due to emissions of greenhouse gases such as carbon dioxide (CO2) and methane (CH4). About a quarter of global human-originated CO2 emissions are currently estimated to come from mobile sources, i.e., automobiles, trucks, buses and trains that are powered by an internal combustion engine (ICE). This proportional contribution is likely to grow rapidly in the foreseeable future with the projected surge in automobile and truck ownership in developing countries. At present, the transportation sector is a major market for crude oil, and controlling CO2 emissions is both an environmentally responsible and a desirable goal in order to maintain the viability of the crude oil market in the transportation sector in the face of challenges from alternative technologies, e.g., cars powered by electric motors and storage batteries.
Carbon dioxide management from mobile sources presents many challenges including space and weight limitations, the inability to achieve economies of scale and the dynamic nature of the operation of the ICE powering the mobile source.
Prior art methods for the capture of CO2 from combustion gases have principally focused on stationary sources, such as power plants. Processes have been developed that use, for example, amines and amine-functionalized liquids and solutions to absorb CO2 at temperatures ranging from ambient up to about 80° C. At temperatures above 100° C., and particularly in the range of from about 130° C. to 600° C. that are encountered in vehicles powered by an ICE, the amines exhibit low capacity for CO2 absorption. Thus, the high temperature of the ICE exhaust gas makes direct treatment to remove CO2 with liquid amine solutions impractical.
Aqueous ammonia has also been used in power plants to capture not only carbon dioxide, but SOx and NOx compounds. The absorption process must be conducted at relatively low temperatures to be effective, so that the solution must be cooled, e.g., to about 27° C. The so-called chilled ammonia process is described in international patent application WO 2006/022885 (2006), the disclosure of which is incorporated herein by reference.
An accepted prior art thermodynamic process used in stationary or fixed sources such as electrical power generation facilities for converting thermal energy into usable mechanical power is the Kalina Cycle. The Kalina Cycle can be implemented in order to increase the overall efficiency of the energy recovered from the fuel source. The process is a closed system that utilizes an ammonia-water mixture as a working fluid to improve system efficiency and to provide more flexibility under varying operating conditions that have cyclical peak energy demand periods. The Kalina Cycle would not be suitable for use on board a mobile source as a separate mechanical energy/work producing system due to the added weight and associated capital expense as compared to Rankine cycle systems.
Historically, the capture of CO2 from mobile sources has generally been considered too expensive, since it involves a distributed system and a reverse economy of scale. The solution to the problem must take into account the practical considerations of on-board vehicle space limitations, the additional energy and apparatus requirements and the dynamic nature of the vehicle's operating cycle, e.g., intermittent periods of rapid acceleration and deceleration.
Some prior art methods that address the problem of reducing CO2 emissions from mobile sources employ sorbent materials that can be subjected to regeneration and reuse of the CO2 capture agent and make use of waste heat recovered from the various on-board sources. Oxy-combustion processes employed with stationary sources using only oxygen require an oxygen-nitrogen separation step which is more energy-intensive than separating CO2 from the exhaust gases and would be more problematic if attempted on board a vehicle.
For purposes of describing the present invention, “mobile source” means any of the wide variety of known conveyances that can be used to transport goods and/or people that are powered by one or more internal or external combustion engines that produce a hot exhaust gas stream containing CO2. This includes all types of motor vehicles that travel on land, as well as trains and ships where the exhaust from the combustion is discharged into a containing conduit before it is discharged into the atmosphere.
As used herein, the term “waste heat” is the heat that a typical internal combustion engine (ICE) produces that is contained principally in the hot exhaust gases (˜300° C. to 650° C.) and the hot coolant (˜90° C. to 120° C.). Additional heat is emitted and lost by convection and radiation from the engine block and its associated components, and other components through which the exhaust gas passes, including the manifold, pipes, catalytic converter and muffler. This heat energy totals about 60% of the energy that typical hydrocarbon (HC) fuels produced when combusted.
As used herein, the term “internal heat exchanger” means a heat exchanger in which the respective heating and cooling fluids originate in the mobile source.
As used herein, “stationary source” means any of the wide variety of known industrial systems and processes that burn carbon-containing fuels and emit CO2 to produce heat, work, electricity or a combination thereof and that are physically fixed.
As used herein, the term “lean loading” means the amount of CO2 remaining in the lean adsorption/absorption solution coming out of the bottom of the CO2 stripper. In accordance with established usage in the field, loading is defined as the moles of CO2 per mole of the amine group or other compound that captures the CO2 by adsorption or relative absorption. As used herein, the terms “CO2-rich solution” and “CO2-lean solution” are synonymous with “rich loaded CO2 solution” and “lean loaded CO2 solution”.
The problem of improving the efficiency of the energy recovered from hydrocarbon fuel combustion in an ICE has been addressed by taking advantage of the waste heat that is present in the engine coolant, the exhaust gas stream and the engine block, manifolds and other metal parts.
Incorporating an energy recovery system requires space, added weight and a specific capital expenditure. However, this investment can be worthwhile if the energy recovery system improves the overall efficiency of the fuel conversion to mechanical power, while reducing the CO2 emissions into the atmosphere, and does this without substantially increasing fuel consumption.
It had long been the practice to use CO2 as a non-toxic and non-flammable refrigerant gas in air conditioning systems prior to the use of chlorofluorocarbon (CFC) refrigerants. It has been proposed more recently in order to improve vehicle efficiency to operate an air conditioning system in reverse, utilizing heat from the vehicle's hot exhaust gas stream to generate additional power for use on board the vehicle. See, e.g., Chen et al., Theoretical Research of Carbon Dioxide Power Cycle Application in Automobile Industry to Reduce Vehicle's Fuel Consumption, Applied Thermal Engineering 25 (2005) 2041-2053. The systems contemplated are closed systems and are based on the moderate value of the critical pressure of CO2. There is no capture and recovery of CO2 from the exhaust gas stream in order to reduce CO2 emissions into the environment.
A so-called thermal engine for power generation has been described that uses waste heat from the flue gases produced by a stationary source in a closed loop system that uses supercritical CO2 (ScCO2) as the working fluid. See Persichilli et al., Transforming Waste Heat to Power Through Development of a CO2-Based Power Cycle, Electric Power Expo 2011 (May 2011) Rosemont, Ill. The ScCO2 passes in heat exchange with hot flue stack gases and then through a turbine where the waste heat is converted to mechanical shaft work to produce electricity. A recuperator recovers a portion of the residual heat and the remainder is discharged from the system through a water or air-cooled condenser, from which the CO2 exits as a subcooled liquid for passage to the pump inlet. Again, this closed system is adapted for integrated use with an industrial heat source to improve the overall efficiency of the associated system. It does not capture CO2 for the purpose of directly reducing its emission into the atmosphere with the exhaust gases.
Incorporating a CO2 capture system on board a mobile source to reduce CO2 emissions adds weight, energy consumption, capital expenditures and maintenance. The problem is to provide a compact system that is easy to operate and maintain at an acceptable and competitive cost of manufacture.
Another problem addressed by the present invention is how to provide an effective and efficient CO2 capture system in combination with an energy recovery and conversion system to produce the electrical and/or mechanical energy needed to compress the CO2 for on-board storage, operate the associated systems and power the mobile source accessories.
A related problem is how to combine the CO2 capture and energy recovery systems to increase the overall efficiency and reduce the number of components, weight, capital expenditure, and maintenance of the overall system and the vehicle.
Technical problems associated with CO2 capture from mobile sources include how to further increase the efficiency of on-board CO2 capture so that operating a conventional ICE powered by hydrocarbon fuels will remain economically and environmentally competitive with the all-electric and hybrid automobiles. These traditional problems are addressed by the processes and systems disclosed, for example in WO/2012/100149, WO/2012/100165, WO/2012/100157 and WO/2012/100182 which integrate CO2 capture, heat recovery and CO2 capture agent regeneration and reuse systems, hereinafter referred to as “multiple systems”. However, utilizing multiple systems in mobile applications also increases weight, energy consumption, capital expenditure, and maintenance associated with operation of the vehicle.
The problem remains of further improving the efficient on-board capture of CO2 from the hot exhaust gas stream from the ICE powering a mobile source.